The soils analysis uses the Water Erosion Prediction Project (WEPP) soil erosion model to analyze impacts to soil resources. WEPP simulates the conditions that affect erosion, such as the amount of vegetation canopy and soil water content, to estimate erosion rates. To facilitate this analysis, the Internet-based U.S. Forest Service (USFS) WEPP interfaces were used for erosion predictions using the “Disturbed WEPP” and “WEPP Road” modules.
Erosion rates are inherently difficult to predict. The rates of erosion predicted by WEPP are within +/-50 percent. Despite this lack of precision, these rates are appropriate for comparing and analyzing impacts of the alternatives on the soil resource. Erosion rates are calculated for different resource programs using surface-disturbance acreage figures as projected in the reasonable foreseeable action table in Appendix T.
WEPP model climate parameters were developed using Worland, Wyoming precipitation data at 5,000 feet above mean sea level to represent the entire Planning Area. Both the Disturbed WEPP and WEPP Road modules are limited to four soil textures (clay loam, silt loam, sandy loam, and loam). The WEPP analysis used a loam soil texture for all erosion predictions.
Disturbed WEPP has eight vegetative treatment options available: 20-year-old forest, 5-year-old forest, shrub-dominated rangeland, tall-grass prairie, short-grass prairie, low-severity fire, high-severity fire, and skid trail. By adjusting cover parameters, these vegetative treatment options can be applied to a wide variety of vegetative communities and land uses.
All WEPP erosion analyses used a 50-year simulation to represent the return interval.
The WEPP analysis used the following parameters:
Slopes used in Disturbed WEPP – Upper slope 0 to 25 percent; lower slope 5 to 25 percent
Slope lengths used in Disturbed WEPP – 300 feet (standard length used for environmental analysis in the Planning Area)
Gradients used in WEPP Road – Road gradient 4 percent; fill gradient 30 percent; buffer gradient 15 percent
Lengths used in WEPP Road – Road length 200 feet; fill length 15 feet; buffer length 130 feet
Width used in WEPP Road – Road width 12 feet
Rock cover used in Disturbed WEPP and WEPP Road – 5 percent
Appendix V provides a full list of the assumptions and parameters used in the WEPP analysis, and a table of erosion rates calculated by resource area.
Other assumptions used in this impact analysis include:
The WEPP model predicts little or no erosion on undisturbed rangelands and forestlands.
Bare soil (without vegetation or other surface cover) with a surface layer that has been altered from its natural condition is more susceptible to accelerated wind and water erosion than undisturbed soil.
Implementing the Wyoming Standards for Healthy Rangelands (Appendix N) improves vegetation health, vigor, cover, and litter, as well as minimizes erosion rates in most areas.
Wind erosion can affect soil productivity in a similar manner as water erosion. Because current soils data is not adequate to make a realistic determination of acres susceptible to wind erosion on rangelands, and there is no wind-erosion prediction technology available for use in a rangeland setting, this analysis will be limited to impacts resulting from water erosion.
Most soils with a moderate water erosion potential within the Planning Area correlate with steep slopes (greater than 15 percent).
For the purposes of this analysis, it is assumed that erosion rates following surface-disturbing activities return to background levels within 3 to 5 years following full reclamation (WEPP 2008).
The BLM will use BMPs to reduce runoff, soil erosion, and sediment yield, and to retain water on the landscape.
To be effective on highly erodible soils, more extensive BMPs than those in common use are required to be utilized and aggressively maintained. The risk of BMP failure is greater on highly erodible soils.
Although some forms of surface disturbance are restricted on slopes greater than 25 percent, it is assumed disturbance on highly erosive soils is distributed across the landscape in the same proportion these soils occur on the land, unless a proposed management action specifies additional protective measures. In other words, if 5 percent of the soils in the Planning Area are highly erosive, then it is assumed that 5 percent of the projected total disturbance would occur on highly erosive soils.
Projected surface disturbance for each alternative potentially modifies soils by disrupting soil stability, changing vegetative cover that can reduce nutrient recycling, damaging biological crusts, decreasing productivity, and increasing compaction. When these modifications occur on highly erodible soils, the potential for accelerated erosion is greater than on less erodible soils. Site-specific erosion predictions and calculations require detailed soil mapping of areas to be disturbed. Soil mapping during site-specific analysis enables the BLM to minimize disturbance of highly erodible or otherwise sensitive soils.
Sensitive soils incur greater adverse impacts from surface-disturbing activities than nonsensitive soils. Sensitive soils are fragile and especially susceptible to adverse impacts from surface disturbance because they are highly erodible and saline, sodic, or alkaline, or have a low reclamation potential. These fragile soils have limited reclamation potential either because of the vegetative community, physical or chemical limitations, susceptibility to erosion, or steep slopes.
Installing and maintaining erosion controls and other mitigation measures, such as BMPs, result in a substantial reduction in soil erosion, depending on site conditions. However, these measures may not reduce adverse soil compaction and productivity impacts.
Subject to applicable laws and regulations, surface‐disturbing activities on fragile soils, soils with low reclamation potential, and soils with highly erosive characteristics will be authorized on a case‐by‐case basis.
Fine-textured soils are more susceptible to water erosion and compaction when wet, whereas coarse-textured soils are more susceptible to wind erosion.
Unless constrained by a management action or other data, surface disturbance will increase throughout the Planning Area during the planning cycle.